Industrial burners utilize a flame detector in a fuel valve control to enable opening of a fuel valve in the presence of a flame or conversely, to close the fuel valve in the absence of a flame. The preferred optical flame detector is an ultraviolet sensitive, cold cathode, gas discharge tube, hereinafter generally referred to as a UV tube.
A UV tube has cathode and anode elements in a gas filled envelope through which ultraviolet rays are transmitted to the cathode element. The cathode emits electrons when exposed to ultraviolet rays, as from a flame. The electrons are accelerated from a negatively charged cathode to an anode charged to the discharge starting voltage and ionize the gas filling the tube by colliding with molecules of the gas, generating both negative electrons and positive ions. The electrons are attracted to the anode and the ions to the cathode, generating secondary electrons. A gas discharge avalanche current flows between cathode and anode. With the UV tube connected in a quenching circuit, the anode/cathode voltage drops below the discharge sustain voltage of the tube and conduction ceases. As charge drains from the quench circuit, the anode/cathode voltage is again sufficient to initiate conduction and the cycle repeats, resulting in a pulse signal at a frequency directly related to the intensity of the ultraviolet rays.
In the absence of UV stimulation, a UV tube will conduct when the anode/cathode voltage exceeds the breakdown voltage VB. Conduction ceases when the anode/cathode voltage drops below the discharge stopping voltage VD.
UV tubes, however, are subject to failure from contamination of the electrodes or the gas. When the UV tube is contaminated, the time for the gas and ions to neutralize increases and the voltage at which conduction occurs is reduced. This results in a higher frequency of operation; and in conduction indicative of a flame in the absence of a flame. It is known to shield the UV tube from the flame intermittently with a mechanical shutter to identify a contaminated tube. Conduction of the tube with the shutter closed indicates tube contamination. Mechanical shutters, however, have moving parts and require frequent maintenance.
UV tubes are also subject to an apparent shifted spectral response (SSR) of reduced signal to noise condition in which the pulse rate increases in the absence of UV radiation. I believe the condition may be due to the deposition of electrode material on the inner wall of the tube which in turn traps some of the gas atmosphere within the tube lowering the gas density and reducing VB and VD. Another cause appears to be contamination in new UV tubes in the form of particles which become electrostatically charged. This condition also reduces VB and VD. In turn, the occurrence of pulse signals in the anode/cathode circuit in the absence of UV radiation increases and the signal to noise ratio of the anode/cathode circuit decreases. Detection of the apparent SSR condition is enhanced by periodically increasing the anode/cathode operating voltage V0.